ECG Interpretation

The electrocardiogram (ECG) records the average of all cardiac action potentials detected as field potentials on the body surface. It records electrical events, not mechanical events. Electrical events always precede the corresponding mechanical events.

ECG polarity depends on the direction of the depolarisation wave relative to the positive electrode. Depolarisation towards a positive electrode produces a positive deflection; depolarisation away from a positive electrode produces a negative deflection. Repolarisation rules are opposite: repolarisation away from positive → positive deflection.

The standard 12-lead ECG uses 10 electrodes: RA, LA, LL (limb leads), and V1-V6 (chest leads). Standard limb leads I, II, III are bipolar. Lead II (+ve on LL, -ve on RA) measures along the heart's electrical axis and shows the classic textbook ECG. Augmented limb leads (aVR, aVL, aVF) are unipolar. Chest leads V1-V6 are unipolar and view the heart in the horizontal plane. ECG paper runs at 25 mm/s: 1 small box = 0.04 s = 0.1 mV; 1 large box = 0.2 s = 0.5 mV.

ECG waves: P wave = atrial depolarisation (slow, 0.5 m/s → fat wave). PR interval (0.12-0.20 s) = conduction from SA node through atria, AV node, His bundle and Purkinje to ventricular onset. Q wave = septal depolarisation (left → right, away from lead II → negative). R wave = main ventricular depolarisation (towards LL → large positive). S wave = final ventricular depolarisation (away → small negative). ST segment = plateau phase of ventricular APs; should be isoelectric — ST elevation indicates myocardial infarction (current of injury). T wave = ventricular repolarisation (epicardium → endocardium, away from LL → positive deflection, and fat as slow).

Heart rate can be calculated from ECG by the 300 method (300 ÷ number of large boxes between R waves), the 1500 method (1500 ÷ small boxes between R waves), or the sequence method (300, 150, 100, 75, 60, 50 assigned to successive large box lines). The 10× method (count R waves in 6 s × 10) is quickest for irregular rhythms.

Chest leads show a transition from dominant S wave (V1-V2, viewing right ventricle) to dominant R wave (V5-V6, viewing left ventricle). The isoelectric lead (equal R and S) is normally V3-V4.